Field-dependent molecular ionization and excitation energies: Implications for electrically insulating liquids

Davari, Nazanin; Åstrand, Per‐Olof; Unge, Mikael; Lundgaard, L.E.; Linhjell, D.

doi:10.1063/1.4869311

Cited by 25 publications

(20 citation statements)

References 74 publications

(67 reference statements)

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“…are greatly diminished [42][43][44]. The field strength shown in figure 18(b) is the local field around the molecule that is higher than the macroscopic field of figure 18(a) by a factor of 2-10 depending on the type of molecules in the liquid [45].…”

Section: Discussionmentioning

confidence: 94%

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Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

et al. 2020

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This study addresses the dielectric performance of nonpolar hydrocarbon liquids and mineral oils under negative polarity stress. Stopping length for non-breakdown streamers, breakdown voltages and velocities for various pre-breakdown streamer modes have been studied for a selection of model liquids (cyclohexane and white oils), for a gas to liquid oil, and a refined naphthenic transformer oil. Studies of propagation modes were done using an 80 mm point to plane gap and a step voltage with 0.5 μs rise time. Light emission and pre-breakdown currents have been recorded and instantaneous velocities have been derived from images of propagating streamers. Compared to positive polarity, there are less differences in streamer behaviour in the oils examined under negative polarity. Breakdown voltages and acceleration voltages are higher for negative streamers than for positive ones, while their propagation velocities are lower. While propagation modes for positive voltages are quite distinct, the mode changes for negative ones are more gradual. The behaviour of both positive and negative streamers is in line with the hypothesis that the propagation is governed by electron avalanches and quantum chemical properties of liquid components.

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Section: Discussionmentioning

confidence: 94%

“…the Laplacian field) and b: Ionization potential of 1st excited state and the ground state versus field in cyclohexane; data extracted from figure 3 in [42]. The field dependence of the ionization potential and several of the excitation energies of cyclohexane have also been calculated in [43].…”

Section: Discussionmentioning

confidence: 99%

Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

et al. 2020

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“…According to Fig. 4, a width of V n * Needle tip curvature r n 6.0 µm Streamer tip curvature [5] r s 6.0 µm Field in streamer [6,51] E s 2.0 kV mm 1 Electron detachment threshold E d 1.0 MV m 1 Avalanche threshold [14] E a 0.2 GV m 1 Scattering constant [14] E ↵ 3.0 GV m 1 Max avalanche growth [14] ↵ m 200 µm 1 Meek constant [8] Q c 23 Electron mobility [42,43] e 45 mm 2 /Vs Anion mobility [11] ion 0.30 mm 2 /Vs Ion conductivity [41] ion 0.20 pS m 1 Base liquid IP [52] I b 10.2 eV Additive IP [53] I a 7.1 eV Additive IP di . factor [8] k ↵ 2.8 eV 1 Additive number density c a,n 0.0 1.5 mm gives about 1 µs before collision, both from the sides and from below.…”

Section: Region Of Interestmentioning

confidence: 99%

Simulation model for the propagation of second mode streamers in dielectric liquids using the Townsend-Meek criterion

et al. 2018

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A simulation model for second mode positive streamers in dielectric liquids is presented. Initiation and propagation is modeled by an electron-avalanche mechanism and the Townsend-Meek criterion. The electric breakdown is simulated in a point-plane gap, using cyclohexane as a model liquid. Electrons move in a Laplacian electric field arising from the electrodes and streamer structure, and turn into electron avalanches in high-field regions. The Townsend-Meek criterion determines when an avalanche is regarded as a part of the streamer structure. The results show that an avalanche-driven breakdown is possible, however, the inception voltage is relatively high. Parameter variations are included to investigate how the parameter values affect the model. visible light [3], re-illuminations, from one or more of its branches. Above the breakdown voltage, streamers may change between the 2nd, the 3rd, and the 4th mode during propagation. There are usually more reilluminations in the 3rd mode than the 2nd mode. The inception of the 4th mode is associated with a drastic increase in speed and fewer, more luminous, branches [2].There are numerous mechanisms that can be involved in the streamer phenomena, the challenge is identifying their importance during initiation and propagation. Applying a potential to a needle can cause charge injection, giving a space-charge limited current [16] causing Joule heating [16], which in turn can cause bubble nucleation [17]. A breakdown in the gas bubble can then propagate the needle potential, and the process may repeat. This is one way to explain 1st mode propagation. Electric fields can also cause electrohydrodynamic flow, which could cause streamer formation through cavitation [18]. Electrostatic cracking has also been proposed as a cavitation mechanism [19]. A main topic of discussion is whether a lowering of the liquid density is needed before charge generation can occur. Electron avalanches are important in gas discharge, but their importance in liquid breakdown is still disputed. In water, strong scattering could prevent electrons from forming avalanches in the liquid phase [20]. Therefore, discharges in micro-bubbles can be important for charge generation [10,14,20]. The same mechanism was also proposed for non-polar liquids [19], however, the relative permittivity is about 80 in water and about 2 in a typical oil, and this difference can prove important since the field enhancement within a bubble in oil is much lower than in water. Contrary to water, there are indications of electron avalanches in non-polar liquids [16,21,22], furthermore, while the initiation and the propagation length of 2nd mode streamers are dependent on the pressure, their propagation velocity is not pressure dependent [16,23]. This implies that the mechanism responsible for propagation occurs in the liquid phase and that the gaseous channel follows as a consequence. In very high electric fields, field-ionization can occur [24,25], and this mechanism has been proposed for the fast 3rd and 4th propagation mode...

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“…Our purpose in this work is to investigate possible ionization effects associated with crosslinking byproducts. Though the matrix is a solid, this has relation to dielectric liquids, as ionization processes appear to be much better understood in liquids than in solids, with as common driving parameter the ionization potential of molecules [7] to explain streamer propagations in liquids [8] and resistance to treeing processes in solids [9,10] for example. The other link to dielectric liquids is the need to maintain the liquid additive into the matrix and control the product uptake.…”

Section: Introductionmentioning

confidence: 99%

Effect of Cumyl Alcohol in Multilayer Dielectric on Space Charge Build Up

Reinmart¹,

Teyssèdre

Roy

et al. 2019

2019 IEEE 20th International Conference on Dielectric Liquids (ICDL)

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One of the issues of using cross-linked Polyethylene (XLPE) in DC insulation systems is the propensity of crosslinking byproducts to promote charge buildup inside the insulation, leading potentially to insulation failure. Field-induced ionization of the molecules, like in insulating liquids, is thought to be one of the processes of charge generation. In this paper, space charge measurement is realized to probe the effect of cumyl alcohol as one of the crosslinking byproducts. Low Density Polyethylene (LDPE) is used as polymer matrix, and soaked in cumyl alcohol. We show that polyethylene naphthalate (PEN) layers constitute efficient barriers to the evaporation of cumyl alcohol from LDPE films. The space charge behavior of soaked LDPE is compared to that of reference LDPE with PEN. Significant charge build-up occurs at the LDPE/PEN interfaces due to the conductivity gradient. PEN appears not suited for such analysis as the field is concentrated in the layer and is weak in the bulk LDPE.

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Field-dependent molecular ionization and excitation energies: Implications for electrically insulating liquids

Abstract: Field dependence on the molecular ionization potential and excitation energies compared to conductivity models for insulation materials at high electric fields

Cited by 25 publications

References 74 publications

Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

Pre-breakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 2: behaviour under negative polarity and comparison with positive polarity

Simulation model for the propagation of second mode streamers in dielectric liquids using the Townsend-Meek criterion

Effect of Cumyl Alcohol in Multilayer Dielectric on Space Charge Build Up

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